1. Field of the Invention
The present invention relates generally to the field of molecular biology. More specifically, the invention relates to plant genes involved in phosphorous uptake and methods of use thereof.
2. Description of the Related Art
Phosphate (Pi) is one of the key substrates in energy metabolism and biosynthesis of nucleic acids and membranes. It also plays an important role in photosynthesis, respiration and regulation of a number of enzymes (Raghothama, 1999). While it is a critical macronutrient for plant growth and development, most of the total soil phosphorus (P) is not available for uptake due to its rapid immobilization by soil organic and inorganic components (Von Uexküll and Mutert, 1995; Whitelaw, 2000). Phosphorus is limiting for crop yield on over 30% of the world's arable land, and by some estimates, world resources of inexpensive rock phosphate may be depleted by 2050 (Vance et al., 2003). The lack of inexpensive P has been recognized as a potential future crisis in agriculture (Abelson, 1999). In consideration of the trend toward sustainability and environmental stewardship, P has been a key nutrient in maintaining long-term productivity of agricultural systems (Iyamuremye and Dick, 1996).
Organic phosphorus plays a vital role in the P cycle of agricultural soils (Dalal, 1977). Anywhere from 30% to 80% of soil P occurs in organic form, which, after mineralization, can contribute considerably to the P nutrition of plants (Bieleski, 1973; Dalal, 1977). Natural efficient acquisition and utilization of organic phosphorus requires a class of endogenous enzymes known as phosphatases (Duff et al., 1994). Acid phosphatase are one form of phosphatase capable of hydrolysing P from orthophosphate monoesters (Duff et al., 1994). One of the adaptive changes of plants under low-Pi conditions is the increased synthesis and secretion of APases (Goldstein et al., 1988; Goldstein et al., 1988; Wasaki et al., 1999; Haran et al., 2000; Wasaki et al., 2000). Plant APases are involved in many biological processes such as providing P during seed germination from stored phytate, internal remobilization of P, release of P from soil organic P-esters and the synthesis of glycolate from P-glycolate (Vance et al., 2003). However, the relative importance of these enzymes for plant P nutrition has yet to be determined (Tomscha et al., 2004).
Although many acid phosphatase genes have been identified in plants based on sequence analysis (Schenk et al., 2000; Li et al., 2002), only a limited number of APase genes have been characterized in any detail (del Pozo et al., 1999; Wasaki et al., 1999; Haran et al., 2000; Baldwin et al., 2001; Miller et al., 2001). The type 5 APase gene (AtACP5) from Arabidopsis (del Pozo et al., 1999) and a Pi starvation-induced APase gene (LePS2) from tomato (Baldwin et al., 2001) were implicated in internal P remobilization. The two genes were highly inducible in roots and shoots under Pi-deficient conditions, while no accumulation of transcripts was detected in either roots or shoots under Pi-sufficient conditions (del Pozo et al., 1999; Baldwin et al., 2001). The transcript of the membrane-bound form of APases (LASAP1) from white lupin (Wasaki et al., 1999) was detected at its highest levels in roots and shoots under Pi-deficient conditions, although much lower levels of transcript were also detectable in roots and shoots under Pi-sufficient conditions (Wasaki et al., 1999; Miller et al., 2001). The mRNAs of the secretory forms of APase from white lupin were only detectable in roots under Pi-deficient conditions, while no transcript was detected in roots and shoots under Pi-sufficient conditions (Wasaki et al., 2000; Miller et al., 2001; Wasaki et al., 2003).
Due to the general phenomenon of APase secretion under P stress and the positive correlation between APase activity and P uptake reported in some studies (Goldstein et al., 1988; Helal, 1990; Asmar et al., 1995; Haran et al., 2000; Wasaki et al., 2000), the role of increased secretion of APase to liberate P from organic sources in the soil has been discussed (Tarafdar and Claassen, 1988; Duff et al., 1994). However, some comparative studies between genotypes or recombinant lines have produced results showing a negative or no relationship between root APase activity and P uptake under Pi stress (McLachlan, 1980; Hunter and McManus, 1999; Yan et al., 2001). For example, it has been shown that a major gene contributing to APase activity in common bean was not associated with P acquisition efficiency and P use efficiency (Yan et al., 2001).
The conflicting results may be due to substantial heterogeneity among APases with regards to their kinetic properties and subcellular locations, and various APases may have distinct metabolic functions. As pointed out by Duff et al. (1994), the diversity and ubiquity of plant APases make a consensus on their precise physiological and biochemical roles difficult to achieve. Perhaps because of this, there has not been any report on improving P uptake by overexpression of plant APase.
While the studies to date have furthered understanding of phosphorous utilization, methods for increasing soil phosphorous uptake have been lacking. There is a great need for the identification of such methods due to the depletion of natural phosphorous sources and because of the significant deleterious effects of phosphorous depletion on agricultural productivity. Applications of phosphorous-rich fertilizers can also create run off polluting water sources. The identification of methods of increasing phosphorous uptake would therefore represent a significant benefit to agriculture and the environment alike.